Input protection circuit for high-speed analogue signal and time-of-flight mass spectrometer

ABSTRACT

The present invention is provided to reduce the distortion or delay of the input waveform of a high-speed analog signal due to the capacitance of a protection element inserted for the purpose of ESD protection in an input circuit for a high-speed analog signal, so as to determine the correct waveform of the high-speed analog signal. In an input protection circuit for a high-speed analog signal, a resistor is provided on a signal path connecting an input connector and an input circuit; a polymer ESD protection element is connected between a portion of the signal path, which extends from the resistor to the input connector, and the ground; a semiconductor ESD protection element is connected between a portion of the signal path, which extends from the resistor to the input circuit, and either the ground or power source; and the sum of the resistance value of an input resistor of the input circuit and the resistance value of the resistor is equal to the characteristic impedance of a high-speed analog signal input.

TECHNICAL FIELD

The present invention relates to a circuit for protecting elements of aninput circuit from an overvoltage due to an electric discharge or otherreasons in a circuit for measuring a high-speed analogue signal. Such acircuit is particularly used as an input protection circuit of atransient recorder, which is a type of waveform-recording circuit fordetermining a signal waveform with the gigaherz (GHz) band.

BACKGROUND ART

The recent increase in the frequency of signals handled by electroniccircuits has generally increased the importance of input protectioncircuits having excellent high-frequency characteristics. An inputprotection circuit is intended for preventing an electronic circuit frombeing damaged by an electrostatic discharge (ESD), which may occur inthe process of handling the electronic circuit. An ESD is a phenomenonin which electric charges that have accumulated in the human body flowinto the entrance (i.e. the input circuit) of the electronic circuitthrough air discharge or direct contact; this flow of electric chargesgenerates a high voltage in the electronic components arranged in theinput circuit and damages them. An electric discharge may also resultfrom a rise in the voltage on the surface of an insulator due to theaccumulation of electric charges caused by the action of an air currentcreated, for example, by a fan for cooling an electronic device.

In a time-of-flight mass spectrometer or similar apparatus, a high-speedanalogue signal is produced by an ion detector provided within ananalyzer section using a high voltage under vacuum, and then fed into awaveform-recording circuit through a coaxial cable or the like (forexample, refer to Patent Document 1). In such an apparatus, if anelectric discharge occurs within the vacuum space, a high-voltage pulseis induced in the path of the analogue signal and reaches thewaveform-recording circuit. Preventing this high-voltage pulse fromreaching and damaging the input circuit is also an objective ofproviding the input protection circuit at the entrance of thewaveform-recording circuit.

As shown in FIG. 1, a conventional input protection circuit uses asemiconductor ESD protection element for the purpose of inputprotection. The semiconductor ESD protection element 13 is provided inthe signal path extending from an input connector 11 to an input circuit12. In the case of measuring a high-speed analogue signal, an inputresistor 14 having a resistance value equal to the characteristicimpedance of the signal path is provided as a terminator at the entranceof the input circuit 12. The semiconductor ESD protection element 13internally includes high-speed, low-capacity diodes, each diode havingone end connected to the signal path and the other to a power source orground. In the example of FIG. 1, one diode is connected to a clampingpower line +V_(CL); if a high-voltage pulse with a positive polarity hasentered the signal path from the input connector 11, this diode willturn to the conducting state and absorb the energy of the high-voltagepulse into the clamping power line +V_(CL), thus preventing thehigh-voltage pulse from reaching the input circuit 12. The example ofFIG. 1 shows another diode, which is connected to another clamping powerline −V_(CL) and similarly prevents a high-voltage pulse with a negativepolarity from reaching the input circuit 12. In order to direct theelectric current resulting from the high-voltage pulse into the clampingpower lines, the voltage of the signal path rises to a voltage thatequals the voltage obtained by adding a forward voltage drop of thediode to the clamping power line +V_(CL) or −V_(CL). However, theresultant voltage will not be so high as to damage the input circuit 12.One or both of the destinations of the semiconductor ESD protectionelement 13 may be connected to the ground in place of the clamping powerline if the amplitude of the analogue signal to be handled is small orthe signal is unipolar. For example, in the case of handling analoguesignals ranging from 0 to 5 V, the ends of the semiconductor ESDprotection element 13 are connected to a 5 V source and the ground,respectively.

As just described, the ESD protection circuit using a semiconductor ESDprotection element shown in FIG. 1 can confine the voltage of the signalpath within a specific voltage range and thus satisfactorily functionsas the protector for the input circuit 12. However, the recent increasein the frequency band of the analogue signals to be handled has causednew problems, i.e. the distortion of the waveform of the analogue signaland the reflection of the analogue signal, due to the capacitance of thesemiconductor ESD protection element 13.

FIG. 2 shows the result of a simulation of the distortion and reflectionof a waveform in the case of feeding a triangular pulse with a rise timeof 200 ps, a fall time of 200 ps and a peak height of 1 V into awaveform-recording circuit through a coaxial cable (delay time: 0.5 ns)having a characteristic impedance of 50Ω. Under the condition that theinput circuit was terminated by an input resistor of 50Ω the calculationwas performed for each of the semiconductor ESD protection elementshaving the capacitances 0 pF, 1 pF, 2 pF and 3 pF, and the calculatedresults are indicated by the square, rhombic, inverted triangular, andtriangular plots, respectively. The triangular wave located in the lowerleft section of FIG. 2 is the pulsed voltage that was originally fedinto the coaxial cable. The upper section of the same figure shows thevoltage waveform created in the input circuit by a 0.5 ns-delayedarrival of the pulse at the waveform-recording circuit. Located in thelower right section of the same figure is the waveform of a reflectedwave, which is returned to the transmitting end through the coaxialcable with an additionally delay of 0.5 ns.

In the case where the capacitance is 0 pF, the triangular wave that hasentered the input circuit maintains its original form, and no reflectiontakes place. However, as the capacitance of the semiconductor ESDprotection element increases, the increase in the voltage of the inputcircuit at the rising portion of the triangular wave becomes moredelayed. As shown, even after the increasing rate of the voltage hasreached a constant value, the voltage of the input circuit is lower thanthat of the input pulse, and this difference in the voltage is returnedto the transmitting end as a reflected wave. An arrival of the inputpulse at the waveform-recording circuit does not immediately cause anincrease in the voltage of the input circuit since the pulse isinitially used to charge up the capacitance of the semiconductor ESDprotection element. For pulses at high frequencies, the input isshort-circuited by the capacitance of the semiconductor ESD protectionelement, so that a reflected wave results. The difference between thevoltage of the input pulse and that of the input circuit produces anelectric current, which is used to charge the capacitance of thesemiconductor ESD protection element. After the steady state is reached,the voltage of the input circuit begins to follow the voltage of theincoming pulse. In the case where the capacitance is 1 pF, the currentrequired for charging the capacitance is 1V×1 pF÷200 ps=5 mA. Thiscurrent is supplied from the 50Ω input resistor and the 50Ω coaxialcable, 2.5 mA each. Accordingly, the voltage appearing in the inputcircuit is 125 mV lower than that of the pulse and, simultaneously, areflected wave with an amplitude of −125 mV results.

The decrease in the voltage becomes larger as the peak voltage of thepulse becomes higher. Furthermore, unlike the triangular wave, thewaveform that is actually measured by the waveform-recording circuitdoes not have a constant slope. Therefore, the difference between thevoltage of the input pulse and the voltage appearing in the inputcircuit changes with time, which causes a distortion or delay of thewaveform.

The widespread use of high-speed digital communication techniques suchas a universal serial bus (USB) or Ethernet in recent years has openedup more opportunities of using low-capacitance ESD protection elementsthat do not affect the device performance at high frequencies. Theseelements are called the polymer ESD protection element. This deviceincludes a polymer film, which breaks down and turns to the conductingstate when a high voltage is applied to it, thereby absorbing the energyof the electrostatic discharge into the ground or power source. Thecapacitance of the polymer ESD protection element can be decreased to aslow as 0.1 pF, thus making it possible to reduce the distortion or delayof the input-pulse waveform to an allowable level. However, polymersdiffer from semiconductors in that they normally behave as insulators;turning such a material to the conducting state by breakdown requires atrigger voltage of 100 V or higher to be temporarily applied to it. Aninput circuit designed for handling signals at high frequencies has arather low withstand voltage and hence is vulnerable to the applicationof an overvoltage. Accordingly, it is necessary to provide an additionalprotection element between the polymer ESD protection element and theinput circuit.

An example of the ESD protection circuit using a polymer ESD protectionelement is shown in FIG. 3. As shown, the polymer ESD protection element35 is connected to an intermediate point of the signal path extendingfrom the input connector 31 to the input circuit 32. Polymer ESDprotection elements are normally a bipolar device and should have theother end connected to the ground. As already explained, a highimpedance element 36 is additionally provided between the polymer ESDprotection element 35 and the input circuit 32 to prevent the triggervoltage from being directly applied to the input circuit 32 and damagingthe input elements. In the USB or other interfaces, due to the signalinput in a differential form, a common mode choke coil, filter elementor the like is used as the high impedance element 36 in order to obtaina high impedance against the discharge energy while maintaining thehigh-frequency characteristic.

However, in the case of analogue signals, it is often impossible toadopt the differential signaling scheme. Using a resistor, coil or thelike as the high impedance element 36 causes mismatching in theimpedance of the signal line and hence a reflection of the signal.Furthermore, since analogue signals to be measured have a broadfrequency band, it is impossible to use a filter having such anarrowband that matches the transmission frequency as in the case of thedigital communication.

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 2006-32207

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The semiconductor ESD protection element has the problem that itscapacitance causes the input waveform to be distorted or delayed andeventually departed from the correct form of the high-speed analoguesignal to be measured. This problem is particularly serious fortime-of-flight mass spectrometers since they determine the mass of ananalysis sample from the arrival time of a pulse. That is, a delay ofthe waveform leads to an error of the mass measurement value, whereas adistortion of the waveform causes an increase in the pulse width andhence a deterioration in the mass measurement resolution. These factorswill significantly deteriorate the performance of the analyzing device.

On the other hand, the low-capacitance polymer ESD protection elementrequires a high trigger voltage and hence cannot be used, in itsoriginal form, as a protection element for broadband analogue signals.

Means for Solving the Problems

In order to solve the aforementioned problems, the present inventionprovides an input protection circuit for a high-speed analogue signal,which is characterized in that: a resistor is provided on a signal pathconnecting an input connector and an input circuit; a polymer ESDprotection element is connected between a portion of the signal path,which extends from the resistor to the input connector, and the ground;a semiconductor ESD protection element is connected between a portion ofthe signal path, which extends from the resistor to the input circuit,and either the ground or power source; and the sum of the resistancevalue of an input resistor of the input circuit and the resistance valueof the aforementioned resistor is equal to the characteristic impedanceof a high-speed analogue signal input.

In one mode of the present invention, the input protection circuit for ahigh-speed analogue signal is further characterized in that the resistoris a pulse-resistant resistor or surge-resistant resistor.

Effects of the Invention

In the input protection circuit for a high-speed analogue signalaccording to the present invention, most of the electrostatic dischargeenergy is absorbed through the low-capacitance polymer ESD protectionelement into the ground and, simultaneously, the trigger voltage thatoccurs in the low-capacitance polymer ESD protection element is clampedto either the power source or ground by the resistor and thesemiconductor ESD protection element, so that no overvoltage can arisein the input circuit. The use of the pulse-resistant resistor orsurge-resistant resistor is aimed at preventing damages to the resistordue to the application of the trigger voltage.

For high-speed analogue signals, the sum of the resistance value of theinput resistance of the input circuit and the resistance value of theresistor is equalized to the characteristic impedance of the high-speedanalogue signal input so as to achieve the impedance matching andthereby prevent the reflection of the high-speed analogue signal. Theresistance value of the input resistor of the input circuit can bedecreased to a value smaller than the characteristic impedance.Therefore, it is possible to reduce the distortion or delay of thewaveform of the high-speed analogue signal due to the capacitance of thesemiconductor ESD protection element to a practically allowable level.The decrease in the gain of the high-speed analogue signal due to thereduction in the resistance value of the input resistor does not causeany problem since it can be compensated for by increasing the gain ofthe input circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an input protection circuit using a semiconductor ESDprotection element.

FIG. 2 shows a calculated example of the distortion and reflection of ahigh-speed pulse waveform by a semiconductor ESD protection element.

FIG. 3 shows an input protection circuit using a polymer ESD protectionelement.

FIG. 4 shows an example of the input protection circuit for a high-speedanalogue signal according to the present invention.

FIG. 5 shows a calculated example of the distortion and reflection of ahigh-speed pulse waveform by an input protection circuit according tothe present invention.

FIG. 6 shows an example of the time-of-flight mass spectrometer using aninput protection circuit for a high-speed analogue signal according tothe present invention.

EXPLANATION OF NUMERALS

11 . . . Input Connector

12 . . . Input Circuit

13 . . . Semiconductor ESD Protection Element

14 . . . Input Resistor

31 . . . Input Connector

32 . . . Input Circuit

34 . . . Input Resistor

35 . . . Polymer ESD Protection Element

36 . . . High-Impedance Element

41 . . . Input Connector

42 . . . Input Circuit

43 . . . Semiconductor ESD Protection Element

44 . . . Input Resistor

45 . . . Polymer ESD Protection Element

47 . . . Resistor

61 . . . Ion Generator

62 . . . Ion Detector

63 . . . Input Protection Circuit

64 . . . Waveform-Recording Circuit

65 . . . Data Processor

66 . . . Control Circuit

BEST MODE FOR CARRYING OUT THE INVENTION

An input protection circuit for a high-speed analogue signal accordingto the present invention is hereinafter described in detail withreference to the drawings.

FIG. 4 shows an example of the input protection circuit for a high-speedanalogue signal. A resistor 47 is inserted in the analogue-signal pathextending from an input connector 41 to an input circuit 42. A polymerESD protection element 45 is connected between a portion of the signalpath, which extends from the resistor 47 to the input connector 41, andthe ground. A semiconductor ESD protection element 43 is connectedbetween a portion of the signal path, which extends from the resistor 47to the input circuit 42, and a power line +V_(CL), and also between thesame portion and a power line −V_(CL). The sum of the resistance valueof an input resistor 44 of the input circuit 42 and that of the resistor47 is equal to the characteristic impedance of the high-speed analoguesignal input.

The polymer ESD protection element 45 should preferably be connected toa point as close to the input connector 41 as possible so that a highcurrent resulting from an electrostatic discharge can be directed intothe ground through a short path. When an electrostatic discharge occurs,the voltage at the point where the polymer ESD protection element 45 isconnected to the signal path temporarily rises to a trigger voltage of100 V or higher. After the polymer ESD protection element 45 is turnedto the conducting state, the voltage falls to a clamping voltage ofseveral tens volts and then gradually declines with the decrease in theenergy of the electrostatic discharge. Accordingly, a pulse-resistantresistor or surge-resistant resistor is used as the resistor 47 toprevent the resistor from being damaged by the application of thetrigger voltage or clamping voltage. The amount of the current flowinginto the resistor 47 and semiconductor ESD protection element 43 isdecreased since most of the electrostatic discharge energy is collectedthrough the polymer ESD protection element 45 into the ground.

Thus, the present configuration allows the use of a relatively smallpackage resistor and hence the selection of a resistor having a goodhigh-frequency characteristic without increasing the inductance orcapacitance of the package. Without the polymer ESD protection element45, a resistor having a good high-frequency characteristic cannot beused as the resistor 47 since most of the electrostatic discharge energywould be consumed by this resistor.

Another advantage is that the decrease in the forward voltage dropacross the diode of the semiconductor ESD protection element 43increases the degree of stability of the voltage-clamping action.

In a measurement circuit for a high-speed analogue signal, thehigh-speed analogue signal is generally inputted through a coaxial cablehaving a characteristic impedance of 50Ω. Accordingly, for the purposeof impedance matching, the resistance value of the resistor 47 and thatof the input resistor 44 of the input circuit 42 are selected so thattheir sum will be equal to 50Ω. In the present example, the resistancevalue of the resistor 47 is set at 40Ω and that of the input resistor 44at 10Ω. This setting causes the gain of the analogue signal to be onefifth the original level. To compensate for this gain reduction, thegain of the input circuit 42 is raised to five times the original valueto obtain a required signal intensity. Since the impedance observed fromthe semiconductor protection element 43 is decreased, the distortion ordelay that may appear in the waveform at the rising portion or otherportions of the signal is reduced.

FIG. 5 shows the result of a calculation performed, as in the case ofFIG. 2, for each of the semiconductor ESD protection elements having thecapacitances 0 pF, 1 pF, 2 pF and 3 pF, with the results indicated bythe square, rhombic, inverted triangular, or triangular plots,respectively. The lower section of FIG. 5 shows the voltage waveform atthe transmitting end of the coaxial cable, whereas the upper sectionshows the voltage waveform in the input circuit. The figure clearlyshows that the voltage drop is smaller than in FIG. 2, and so is theamplitude of the reflected wave. It should be noted that, for thepurpose of comparison with FIG. 2, the peak height of the input pulse atthe transmitting end in FIG. 5 has been multiplied by five since thesignal intensity in the input circuit 42 is reduced to one fifth as aresult of the aforementioned setting of the resistance ratio of theresistor 47 and the input resistor 44.

In actual measurement circuits for high-speed analogue signals, it isimpossible to extremely increase the gain because it will cause otherproblems, such as an increase in the noise level. However, it ispossible to select an appropriate gain corresponding to the frequencyband or signal waveform of the analogue signal to be measured, wherebyan improvement effect corresponding to the selected gain can beobtained.

Thus, the input protection circuit for a high-speed analogue signalaccording to the present invention reduces the distortion or delay ofthe input waveform due to the capacitance of the protection elementinserted for the purpose of ESD protection, so that the waveform of thehigh-speed analogue signal can be correctly determined.

The input protection circuit for a high-speed analogue signal accordingto the present invention can be used for the protection of awaveform-recording circuit of a device that handles signals at highfrequencies, such as a time-of-flight mass spectrometer. FIG. 6 shows anapplication example for the time-of-flight mass spectrometer. Ions aregenerated in an ion generator 61 at points in time determined by acontrol circuit 66. The generated ions fly through a vacuum space towardan ion detector 62, each ion taking a specific length of timecorresponding to its mass-to-charge ratio (mass/charge) to arrive at thedetector 62. The arrival of an ion causes the ion detector 62 to producean analogue signal, which is transmitted to a waveform-recording circuit64 through an input protection circuit 63 for a high-speed analoguesignal according to the present invention. The signals obtained by thewaveform-recording circuit 64 are collected by a data processor 65,which creates a mass spectrum.

If an electric discharge should occur in the vacuum and induce ahigh-voltage pulse in the analogue-signal path, the input protectioncircuit 63 for a high-speed analogue signal, which is located betweenthe ion detector 62 and the waveform-recording circuit 64, prevents thehigh-voltage pulse from reaching the waveform-recording circuit 64.Although the input protection circuit is provided between the iondetector and the waveform-recording circuit, the distortion or delayobserved in the waveform at the rising portion of the signal is reducedsince the input protection circuit according to the present inventionhas a low impedance when observed from the semiconductor protectionelement. Thus, the present invention can reduce the error of massmeasurement values and improve the mass resolution.

It should be noted that the previously described embodiment is a mereexample of the present invention; any change or modification that isappropriately made within the spirit of the present invention willevidently fall within the scope of the present invention.

INDUSTRIAL APPLICABILITY

The input protection circuit for a high-speed analogue signal accordingto the present invention can be used in any device with a circuit thathandles signals at high frequencies. For example, it can be used toprotect the waveform-recording circuit of a time-of-flight massspectrometer.

The invention claimed is:
 1. A time-of-flight mass spectrometer,comprising: an ion detector that produces a high-speed analogue signal;a waveform recorder for collecting the high speed analogue signalproduced by the ion detector, and an input protection circuit for ahigh-speed analogue signal provided between the ion detector and thewaveform recorder, the input protection circuit including: an inputconnector; an input circuit; a signal path resistor provided on a signalpath connecting the input connector and the input circuit; a polymer ESDprotection element connected between a portion of the signal path, whichextends from the signal path resistor to the input connector, and aground; and a semiconductor ESD protection element connected between aportion of the signal path, which extends from the signal path resistorto the input circuit, and either the ground or a power source, where asum of a resistance value of an input resistor of the input circuit anda resistance value of the signal path resistor is equal to acharacteristic impedance of the high-speed analogue signal input, theresistance value of the input resistor is less than the resistance valueof the signal path resistor such that a gain of the high-speed analoguesignal is reduced, and a gain of the input circuit is increased tocompensate for the reduction in the gain of the high-speed analoguesignal.
 2. The time-of-flight mass spectrometer according to claim 1,which is further characterized in that the resistor is a pulse-resistantresistor or surge-resistant resistor.
 3. The time-of-flight massspectrometer according to claim 1, wherein the polymer ESD protectionelement is connected to a point close to the input connector so that noadditional element is placed between the connector and the polymer ESDprotection element and a high current resulting from an electrostaticdischarge can be directed into the ground through a short path.
 4. Thetime-of-flight mass spectrometer according to claim 2, wherein thepolymer ESD protection element is connected to a point close to theinput connector so that no additional element is placed between theconnector and the polymer ESD protection element and a high currentresulting from an electrostatic discharge can be directed into theground through a short path.